102  
FXUS02 KWBC 270650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
250 AM EDT WED JUN 27 2018  
 
VALID 12Z SAT JUN 30 2018 - 12Z WED JUL 04 2018  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
GUIDANCE MAINTAINS SIMILAR IDEAS FOR THE LARGE SCALE PATTERN  
EVOLUTION THROUGH THE PERIOD ALBEIT WITH SOME EMBEDDED DETAIL  
SPREAD. CYCLONIC FLOW ACROSS THE NORTHERN TIER WILL BROADEN  
DURING THE WEEKEND AS AN EJECTING WESTERN SHORTWAVE PROGRESSES  
EAST AND THEN NORTHEAST FOLLOWED BY WEAK UPSTREAM ENERGY FLOWING  
AROUND A STRONG EASTERN PACIFIC RIDGE. THEN A GULF OF ALASKA  
UPPER LOW WILL DROP SOUTHEASTWARD TOWARD NORTHERN WA/SOUTHERN BC.  
THIS FEATURE COMBINED WITH A RETROGRESSION OF THE PACIFIC RIDGE  
WILL LEAD TO DEVELOPMENT OF A MEAN TROUGH NEAR THE WEST COAST NEXT  
WEEK. FLOW ON THE WESTERN SIDE OF THE TROUGH MAY START TO PULL  
THE OVERALL TROUGH AXIS A LITTLE OFFSHORE AROUND MIDWEEK OR SO.  
THIS WOULD BE CONSISTENT WITH TELECONNECTIONS RELATIVE TO THE  
POSITIVE ANOMALY CENTER ASSOCIATED WITH THE NEW POSITION OF THE  
PACIFIC RIDGE. MEANWHILE THE CENTER OF THE EASTERN U.S. UPPER  
RIDGE RESPONSIBLE FOR THE HEAT WAVE OVER THE EASTERN HALF OF THE  
COUNTRY WILL TRACK EAST-NORTHEAST FROM THE OHIO  
VALLEY/APPALACHIANS. THEN UPPER RIDGING WILL LIKELY BUILD OVER  
THE PLAINS/MS VALLEY/SOUTHERN ROCKIES. AROUND THE PERIPHERY OF  
THESE RIDGES THERE MAY BE SOME DIFFUSE UPPER ENERGY NEAR THE GULF  
COAST AND A WESTERN ATLANTIC WEAKNESS/UPPER LOW THAT MAY  
EVENTUALLY APPROACH THE EAST COAST.  
 
OVER THE PAST DAY GUIDANCE HAS MAINTAINED FAIRLY GOOD CONTINUITY  
IN PRINCIPLE FOR NORTHERN TIER FLOW DURING THE FIRST HALF OF THE  
PERIOD. THERE ARE STILL PERSISTENT DETAIL DIFFERENCES THAT ARE  
SMALL ENOUGH IN SCALE TO HAVE FAIRLY LOW PREDICTABILITY, FAVORING  
A CONSENSUS BLEND AT THIS TIME. LEADING SHORTWAVE ENERGY WILL  
PUSH AN UPPER MS VALLEY-CENTRAL PLAINS FRONT INTO THE GREAT LAKES  
SAT-MON. THE FRONT MAY DECELERATE FOR A TIME EARLY NEXT WEEK AS  
HEIGHTS ALOFT BECOME NEARLY PARALLEL TO THE FRONT. TOWARD THE END  
OF THE PERIOD GUIDANCE STARTS TO SIGNAL A HINT OF TROUGHING ALOFT  
IN THE VICINITY OF EASTERN CANADA/NEW ENGLAND. SPREAD IS  
CONSISTENT WITH DETAIL UNCERTAINTY UPSTREAM, THOUGH  
TELECONNECTIONS RELATIVE TO THE PACIFIC/WEST COAST PATTERN DO  
FAVOR EVENTUAL DEVELOPMENT OF MODERATE TROUGHING OVER EASTERN  
CANADA/NEW ENGLAND. THUS THE SURFACE FRONT REACHING THE EAST  
COULD SPEED UP A BIT AROUND MIDWEEK.  
 
RECENT GUIDANCE TRENDS FOR THE GULF OF ALASKA UPPER LOW HAVE GONE  
FAIRLY STRONGLY TOWARD THE ECMWF/ECMWF MEAN WHICH HAVE BEEN ON THE  
FASTER SIDE OF THE ENVELOPE OVER THE PAST COUPLE DAYS. NEW 00Z  
SOLUTIONS FURTHER THIS TREND WITH THE GFS/CMC NOW SHOWING EJECTION  
OF THE UPPER LOW BY DAY 7 WED THOUGH NOT QUITE AS FAST AS ECMWF  
RUNS. GUIDANCE SPREAD THROUGH THE 18Z CYCLE RECOMMENDED LEANING  
MORE TOWARD THE ENSEMBLE MEANS TO DOWNPLAY UNCERTAIN DETAILS BY  
DAYS 6-7 TUE-WED.  
 
DIFFUSE NATURE OF ENERGY FORECAST OVER THE WESTERN ATLANTIC AND  
GULF COAST LENDS ITSELF TO LOW PREDICTABILITY FOR SPECIFICS.  
THERE HAS BEEN A CONSISTENT SIGNAL FOR GULF COAST ENERGY IN SOME  
FORM OVER THE PAST COUPLE DAYS, WHILE THE GFS/ECMWF HAVE TRENDED  
TOWARD HOLDING WESTERN ATLANTIC ENERGY FARTHER EASTWARD.  
 
ABOVE FORECAST CONSIDERATIONS YIELDED A BLEND OF OPERATIONAL MODEL  
GUIDANCE (18Z GFS AND 12Z ECMWF/UKMET/CMC) DAYS 3-4 SAT-SUN  
FOLLOWED BY A STEADY TREND TOWARD MORE 18Z GEFS AND 12Z  
NAEFS/ECMWF MEAN INPUT SUCH THAT ENSEMBLES CONTRIBUTED 70 PERCENT  
TO THE FORECAST BY DAY 7 WED.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
EXPECT POTENTIAL FOR SOME HEAVY/STRONG CONVECTION BETWEEN THE  
UPPER GREAT LAKES AND CENTRAL PLAINS EARLY IN THE PERIOD, IN  
ASSOCIATION WITH A COLD FRONT INITIALLY NEAR THIS AREA.  
PROGRESSION OF THE FRONT WILL EVENTUALLY BRING  
SHOWERS/THUNDERSTORMS INTO THE EAST BUT WITH ANY HEAVY ACTIVITY  
TENDING TO BE MORE LOCALIZED AS THE BEST DYNAMICS LIFT INTO CANADA  
AFTER THE WEEKEND. THE TRAILING PART OF THE FRONT WILL STALL OVER  
THE PLAINS AND LIFT BACK AS A WARM FRONT SO THERE WILL BE  
POTENTIAL FOR ADDITIONAL RAINFALL OVER PORTIONS OF THE CENTRAL  
U.S. AFTER THE MORE CONCENTRATED ACTIVITY DURING THE WEEKEND. THE  
OTHER AREA OF FOCUS FOR HEAVY RAINFALL POTENTIAL WILL BE OVER THE  
GULF COAST/FLORIDA, WITH THE COMBINATION OF UPPER LEVEL ENERGY AND  
SURFACE TROUGHING CONTRIBUTING TO ACTIVITY.  
 
IN TERMS OF HIGHEST TEMPERATURE ANOMALIES, THE CORE OF THE HEAT  
WAVE AFFECTING THE EASTERN HALF OF THE U.S. SHOULD EXTEND FROM THE  
MIDWEST/GREAT LAKES INTO THE NORTHEAST. THE GREATEST COVERAGE OF  
PLUS 10-20F ABOVE NORMAL READINGS WILL LIKELY EXIST SAT-MON,  
FOLLOWED BY GRADUAL MODERATION AS THE PLAINS FRONT APPROACHES.  
BEST POTENTIAL FOR RECORD HIGHS WILL BE FROM THE LOWER GREAT  
LAKES/UPPER OHIO VALLEY INTO THE NORTHERN MID ATLANTIC/NORTHEAST  
SAT-SUN WHILE BROADER COVERAGE OF RECORD WARM LOWS IS POSSIBLE  
SAT-MON. THE NORTHERN ROCKIES/HIGH PLAINS WILL SEE BELOW NORMAL  
HIGHS DURING THE WEEKEND FOLLOWED BY A TREND TOWARD NEAR NORMAL  
TEMPERATURES. THE EVOLVING PATTERN BY LATE IN THE PERIOD WILL  
TEND TO SUPPORT MODERATELY BELOW NORMAL TEMPERATURES ALONG THE  
WEST COAST AND OVER THE NORTHWEST TOWARD MIDWEEK.  
 
RAUSCH  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES ARE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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